In recent years, with progress in speed-up and high-densification for an optical disc, accuracy improvement in optical servo for maintaining a focal point of a laser beam on an information recording track of the disc in an optical disc device has been rapidly demanded. As a means for improving the accuracy of optical servo, repetitive control (learning control) has attracted attention. The repetitive control controls, with a one-period previous deviation signal being stored in a memory, a control system according to the stored signal.
When performing such repetitive control, however, since the signal of just-previous one period is stored in the memory, if a non-periodic signal caused by flaws on the disc or disturbance such as vibration applied to the device is undesirably given as an input signal, unnecessary noise might be mixed into the control system by learning this signal. Accordingly, there have conventionally been demanded a control system which can perform stable control even when such disturbance is applied, and a repetitive control method which can eliminate the influence of the unnecessary non-periodic component that might be undesirably learned by the memory.
As a means to solve the above-mentioned problem, Patent Document 1 discloses a cyclic memory which constitutes an input signal to a learning memory which includes a positive-feedback loop provided for repeatedly memorizing an input signal for one period, by a present signal that is multiplied by a gain element k (0≦k≦1) and an one-period previous output of the learning memory 4 that is multiplied by a gain element 1−k, and makes the information inside the learning memory, by the value of k, function so as to be not only the one-period previous information but also the information over many periods which are weighted.
FIG. 6 is a diagram illustrating the construction of the cyclic memory included in the conventional optical disc device which is disclosed in Patent Document 1.
In FIG. 6, reference numeral 1 denotes a first adder for adding a compensation target signal having a periodic component such as an error signal in the control system of the optical disc device, which is a target to be followed, and an output of the cyclic memory. Reference numeral 2 denotes an attenuation gain β for varying the degree of learning. Reference numeral 3 denotes a low-pass filter, numeral 4 denotes a memory for storing frequency components for one rotation period of the disc, and numerals 5 and 6 denote gain elements for switching the signals to be stored in the memory 4. Reference numeral 7 denotes a second adder, and an output of this second adder 7 is stored in the learning memory 4. Reference numeral 12 denotes a correlation detection unit for judging whether the input compensation target signal has a periodicity or not, and it is constituted by a low-pass filter 8, a subtracter 9, an absolute value detector 10, and a comparator 11.
In the cyclic memory having the above-described construction, initially, the correlation detection unit 12 judges whether the inputted compensation target signal having a periodic component has a periodicity or not, and detects whether an error signal having no correlation which is caused by such as noise, disturbance, or flaws on the disc surface is superposed on the compensation target signal or not. When the correlation detection unit 12 judges that there is a correlation (no noise detected), the value of k of the gain element 5,6 is set at k=1, while when it is judged that there is no correlation (noise detected), the value of k of the gain element 5,6 is set at k=0.
Then, the inputted compensation target signal and the output signal of the gain element 2 are added by the first adder 1, and the resultant signal becomes an output of the cyclic memory. The output of the cyclic memory is multiplied by k in the gain element 6, and the k-multiplied signal and a value obtained by multiplying the output of the learning memory 4 by 1−k in the gain element 5 are added in the second adder 7, and the resultant signal is input to the learning memory 4. The output of the learning memory 4 is input to the low-pass filter 3, and multiplied by the gain β that is not larger than 1 in the gain element 2 to be fed back to the first adder 1. Thereby, the long-term periodic component can be stored.
When k=1 is outputted from the correlation detection unit 12, the signal equivalent to one rotation of the disc, which is outputted from the first adder 1, is stored in the learning memory 4 via the gain element 6, and the signal stored in the learning memory 4 is fed back to the adder 1 via the low-pass filter 3 and the gain element 2 to satisfy the stability condition in the repetitive control. On the other hand, when k=0 is outputted from the correlation detection unit 12, it is stopped to store the signal equivalent to one disc rotation that is outputted from the first adder 1 into the learning memory 4 by the gain element 6, and the signal equivalent to one disc rotation that has just previously been stored in the learning memory 4 is again stored via the gain element 5, and the stored signal is fed back to the adder 1 via the low-pass filter 3 and the gain element 2.
By adopting the above-described construction, even when a disturbance or the like is mixed into the compensation target signal, the followability of the laser beam can be enhanced without attenuating the degree of learning. In the optical disc device having such repetitive control (learning control) system, since the followability to the periodic target can be enhanced without increasing the control band relative to the focus/tracking control comprising the direct feedback control, it is possible to deal with a system having narrow tracks, a system having a large eccentricity, and a system having a high disc rotation speed (a system having a high transfer rate).
Patent Document 1: Japanese Published Patent Application No. Hei. 9-50303